One of the key concepts of athletic training is specificity of training. The training activity most appropriate to achieving optimal swimming performance is that of swimming at competition or maximal speeds. Since that level of performance can only be maintained for very short periods of time, external assistance is required for extended training periods.
Recently, a sophisticated apparatus for swim instruction, training, and assessment permitted the implementation of this coaching principle in practice (see my U.S. Pat. No. 5,391,080).